Clinical trials are studies performed on human subjects. Conducting trials and applying the results to large populations requires specific methods. The methods linked to clinical trials are presented in Unit 1 “Medical research an development”. Clinical trials are conducted when experimentation on humans is absolutely necessary to answer a specific question and must be carried out in accordance with ethical principles. Ethical questions linked to clinical trials are discussed in Unit 2 “Ethics”. Some basic aspects of statistics and their applications to questions related to clinical trials are presented in this unit.